RAILWAY CONSTRUCTION. 271 



described forms a very strong roof, and is light in cost and 

 maintenance. The weight of ironwork, both wrought and cast, 

 in the principals, arched wrought-iron girders, cast-iron columns, 

 centre gutters, etc., is only 0'51 of a ton per square (of 100 

 square feet) of area covered. For comparison, the weight of 

 ironwork in the roof, Fig. 402, of 198 feet span is 1'42 ton per 

 square of area covered ; and of the roof, Fig. 404, of 210 feet span, 

 is 2-07 tons per square. 



This increase in weight per square as the spans go on 

 increasing results, not only in a much larger outlay for original 

 construction, but entails also a proportionally heavier expendi- 

 ture for maintenance and painting. The item of painting alone 

 is an expensive one in all iron-roof work, and must be attended 

 to regularly for the proper protection and appearance of the 

 ironwork. With the smaller spans, the roof-trusses form very 

 convenient supports for painters' scaffolding or planking, but 

 with the very large spans the greater height and the form 

 of the roof-principals render specially designed scaffolding and 

 appliances necessary for the painting and repairs. 



Doubtless there is something very attractive about a large 

 span roof, its bold outline stretching from side to side of a wide 

 covered area imparts an imposing effect which cannot be claimed 

 for smaller or more moderate spans ; but where roofs are con- 

 structed for purely utilitarian purposes it becomes a question 

 worthy of grave consideration whether a series of smaller spans 

 would not provide the same practical benefits as would be 

 obtained from one very large span. Upon referring to the 

 typical sketch of a terminal station, Fig. 373, it will be seen 

 that the total width from inside to inside of main walls is 

 240 feet. The lines and platforms are so arranged that by 

 placing rows of columns at A, A, B, B, and C C, the entire width 

 may be divided out into four spans of 60 feet ; or, if preferred, a 

 row of columns at B, B may be adopted, resulting in two spans 

 of 120 feet, or the entire width may be included in one large span 

 of 240 feet. Any one of the three arrangements will provide an 

 effectual roof-covering, and the selection must be decided by the 

 cost or expediency. 



Another way to avoid the introduction of large span-roof 

 principals, and to preserve the covered area free from intervening 

 columns, is to erect strong truss-girders extending across at right 

 angles from the main walls. These truss-girders are placed at 



